Most commercially viable processes for the treatment of zinc concentrates in the recovery of zinc and other valuable metals utilize the basic electrolytic zinc process. The steps involved in processing zinc concentrates via the electrolytic process include: roasting of the zinc concentrates to eliminate sulfur and produce zinc calcine; leaching the zinc calcine to produce an impure zinc sulfate solution; precipitating the iron that may be present; purifying the zinc sulfate solution to produce a "pure" electrolyte free of deleterious elements; and recovering the refined zinc metal by electrolysis.
In the above-described roasting step, three major zinc compounds are produced from the original sulfide mineral: zinc oxide (ZnO), zinc ferrite (ZnO.Fe.sub.2 O.sub.3), and zinc sulfate (ZnSO.sub.4), representing respectively approximately 75%, 23% and 2% of the total zinc as in the case of high-iron (10% Fe) zinc concentrates.
The objective of the overall leaching step is to dissolve or "leach" as much of the zinc in the calcine as possible, minimize the harmful impurities in the solution, while at the same time obtaining a pulp which can be easily thickened and filtered. Dilute sulfuric acid (spent electrolyte) is typically used as the solvent for leaching.
With respect to the solubility of the various zinc compounds, whereas zinc oxide is soluble in dilute sulfuric acid according to the reaction: ZnO + H.sub.2 SO.sub.4 .fwdarw.ZnSO.sub.4 + H.sub.2 O, and zinc sulfate is soluble in water, zinc ferrites require more intense leaching conditions, to wit, high temperature and high acid concentrations, than does the zinc oxide and as such it is soluble according to the reaction: ZnO.Fe.sub.2 O.sub.3 + 4H.sub.2 SO.sub.4 .fwdarw.Fe.sub.2 (SO.sub.4).sub.3 + ZnSO.sub.4 + 4H.sub.2 O.
Thus, it can be seen that the leaching step may be broken down in two parts, the first being neutral leaching and the second hot acid leaching, viz. the zinc oxide and zinc sulfate are dissolved in the neutral leach and the zinc ferrites are dissolved in hot acid leach. The purpose of the neutral leach is to dissolve as much zinc oxide as possible from the calcine so as to produce a suitable solution for subsequent purification and to also produce a pulp that can be easily filtered or thickened. The objective of the hot acid leach is to dissolve as much of the remaining zinc (zinc ferrites) as is economically feasible.
Because of the difficulties of removing the iron entering the system in a readily filterable form, most electrolytic zinc plants, at least until recent years, have utilized only the neutral leaching step. This problem has now been overcome, to a certain extent, with the advent of the "jarosite process" and the "geothite process", whereby the iron in solution is removed as a readily filterable precipitate. As noted, the purpose of such iron removal step is to remove the iron introduced into the system from the hot acid leach solution (leaching of zinc ferrites) while minimizing zinc losses. Normally, the solution produced from the iron removal step is then returned back to the neutral leach step for recovery of the contained zinc with the iron-containing precipitate being discarded. A conventional electrolytic zinc process employing a known iron precipitation step is depicted in FIG. 1.
Three major viable methods of removing the iron from acid leach solutions are presently in commercial operation throughout the world in major zinc refineries. These may be designated as the "jarosite process", the "Outokumpu conversion process", and the "goethite process." These processes, in so far as the iron removal steps are concerned, are schematically depicted in FIGS. 3, 5 and 6.
The "jarosite process", named after the resulting iron precipitate, is described in U.S. Pat. Nos. 3,434,947 and 3,493,365. A detailed description of this process, and its various modifications, is also described in an article entitled "Improved Leaching Technologies In The Electrolyte Zinc Industry", Metallurgical Transactions B, Vol 6 B, March 1975, pages 43-53. As shown in FIG. 3, this process, which is the most widely used of the three methods for the removal of iron, consists of adding a small amount of ammonium, sodium or potassium ion, as in the form of ammonium hydroxide, to the acid leach solution, followed by the neutralization of the solution to an acid equivalent of less than about 10 gms/liter (g/l) H.sub.2 SO.sub.4. Neutralization may be carried out with any convenient neutralizing agent, and most preferably with calcine as shown in FIG. 3. Under such conditions, the iron that was present in the solution is precipitated out as jarosite, (NH.sub.4,Na,K)Fe.sub.3 (SO.sub.4).sub.2 (OH).sub.6, according to the following reaction: EQU 3Fe.sub.2 (SO.sub.4).sub.3 + 2(NH.sub.4,Na,K)OH + 1OH.sub.2 O.fwdarw.2(NH.sub.4,Na,K)Fe.sub.3 (SO.sub.4).sub.2 (OH).sub.6 + 5H.sub.2 SO.sub.4
the resulting pulp is then filtered to yield a filtrate and a jarosite precipitate residue. The filterate is recirculated back to the neutral leaching step and the residue is discarded.
The "Outokumpu conversion process", named after the developers of this iron removal process, is described in U.S. Pat. No. 3,959,437. As depicted in FIG. 5, this process in effect combines the hot acid leaching step, in the treatment of zinc calcines, with the subsequent iron removal or jarosite precipitation step. As such, this process is particularly applicable for the treatment of zinc calcines which perchance contain no significant amounts of other metal values such as lead, silver, gold, etc., since under these conditions there is no need to filter after the hot acid leach stage so as to produce a salable residue. The overall reaction of the "Outokumpu conversion process" may be depicted as follows:
______________________________________ Ferrites 3ZnO . Fe.sub.2 O.sub.3 + (7 - X)H.sub.2 SO.sub.4 + X(NH.sub.4).sub.2 SO.sub.4 + (2 - 2X)H.sub.2 O.fwdarw. 2(NH.sub.4) (H.sub.3 O).sub.(1 - x) [Fe.sub.3 (SO.sub.4).sub.2 (OH).sub.6 ] + 3ZnSO.sub.4 Jarosite ______________________________________
The essential difference between the "Outokumpu conversion process" and the "jarosite process" is that in the "Outokumpu conversion process" dissolution of the zinc ferrite, contained in neutral leach residues, and the precipitation of the dissolved iron as jarosite, are carried out simultaneously (See FIG. 5). In the jarosite process, on the other hand, these two operations are carried out in sequence (See FIG. 3). Moreover, by means of prior addition of ammonia to the circuit and careful control of spent electrolyte additions, the "Outokumpu conversion process" may be carried out under conditions such that the dissolution of iron from zinc ferrite and its subsequent precipitation as jarosite are combined and carried out at high acidity, i.e., at an acid content of about 30-50 g/l H.sub.2 SO.sub.4 as compared to an acid content of about 5-10 g/l H.sub.2 SO.sub.4 for the "jarosite process". Thus, the addition of a neutralizing agent (zinc calcine) is not required and higher zinc extractions are obtained.
The "goethite process", likewise named after the resulting iron precipitates, is described in U.S. Pat. No. 3,652,264 and also discussed in the article entitled "Improved Leaching Technology In The Electrolytic Zinc Industry". This process, as depicted in FIG. 6, removes iron from the acid leach solution by precipitating the iron as goethite (FeOOH). Unlike the relatively simple "jarosite process", the "geothite process" is rather involved and consists of (1) reduction of the ferric iron in the acid leach solution to the ferrous state by reaction with sulfide zinc concentrate; (2) filtration of the pulp; (3) preneutralization of the filtrate with zinc calcine to a pH of approximately 2; and (4) oxidation of the ferrous iron with air or oxygen, back to the ferric state, while maintaining the pulp at a ph of approximately 2 with staged additions of calcine. A crystalline, readily filterable goethite iron precipitate is formed which is filtered off and discarded. The reduction and precipitation of iron occurs as follows: EQU Fe.sub.2 (SO.sub.4).sub.3 + ZnS.fwdarw.2FeSO.sub.4 + ZnSO.sub.4 + S EQU 2feSO.sub.4 + 3H.sub.2 O + 1/20.sub.2 .fwdarw.2FeO(OH) + 2H.sub.2 SO.sub.4
with respect to the "jarosite process", this process has the obvious disadvantage that calcine, which is added during the iron precipitation step, results in a carryover of the zinc ferrite in this calcine to the discarded jarosite precipitate. This leads to a lower overall zinc extraction. Moreover, it has been found by applicants in the course of extensive laboratory and pilot plant testing that overneutralization of the jarosite pulp with calcine could become troublesome; that is, it has been found very difficult to determine and control the precise amount of calcine that is to be added such that there is a tendency to add an excessive amount of calcine in order to ensure adequate precipitation of the jarosite. Not only does overneutralization result in poor filtration characteristics, but it also lead to increased zinc losses in the residue due to increased amounts of zinc ferrites remaining in the residue.
Concerning the foregoing disadvantage of the "jarosite process", and particularly in respect to the carryover of valuable zinc ferrite, a "jerosite acid wash process" has been proposed. This process is depicted in FIG. 4 and is also the subject of U.S. Pat. No. 3,684,490 and is likewise described in the article entitled "Improved Leaching Technology In The Electrolyte Zinc Industry". In the "jarosite acid wash process", the precipitated jarosite and unreacted zinc ferrites (from the calcine added for neutralization) are releached with spent electrolyte under similar conditions as those in the hot acid leach. This is possible due to the low resolubility of jarosite in acid solution once it has been precipitated. However, the "jarosite acid wash process" of course has the inherent disadvantages of necessitating additional reaction vessels as well as another solid-liquid separation step.
Likewise, tests performed in respect to the "Outokumpu conversion process" established the following inherent drawbacks: (1) long reaction times and high reacton temperatures are required (24 hours at 95.degree. C.); (2) when the resulting filtrate, which is high in iron (about 10 g/l Fe vs about 1 g/l Fe for the "jarosite process") is recycled back to the neutral leaching step, the additional iron present creates extreme settling and filtration problems.
Similar laboratory test work conducted by applicants upon the "geothite process" has clearly demonstrated that long reaction times are necessary (approximately 16 hours) and that the reaction must be carried out at high temperatures (approximately 95.degree. C.) in order to conduct the complex series of operations required in this process. Moreover, the process is subject to sensitive pH conditions and the further requirement of careful control during stage additions of calcine.
Thus it can be seen that all of the commercially viable processes presently known for the precipitation of dissolved iron in the electrolytic recovery of zinc have certain inherent disadvantages; these may be summarized as follows: (1) the addition of calcine to the jarosite pulp in the "jarosite process" results in high zinc losses since zinc ferrites contained in the calcine added for neutralization are not efficiently dissolved and may result in filtration problems; (2) the "jarosite acid wash process" (for recovery of the foregoing zinc ferrites), requires additional equipment as well as extra process steps; (3) the "Outokumpu conversion process" requires long reaction times at high temperatures and results in the recycle of high quantities of iron; and (4) the "goethite process" is rather complex and requires sensitive control.